The Breitling Slide Rule Bezel

In a time of calculators, computers, smart phones and the internet, the answer to any question is never more than a few taps of a screen or clicks of a button away. Remember when you had to just know who was in a movie, or what that song was? Before then, before all of that, there was a time when complex calculations were only achievable by hand or with a slide rule. It’s a skill that’s been lost over generations, but today we’re going to find out how, exactly, this mess of lines and numbers actually works.

The famous Navitimer slide rule, designed by pilots for pilots, has become a trademark style for Breitling—but it didn’t actually originate on the Navitimer at all. It wasn’t even originally made for pilots, either. It was 1934 when Willy Breitling, son of founder Léon Breitling, filed a patent for the first chronograph design with not one but two pushers, splitting the start-stop function from the reset for the first time. This set him off on a quest to produce the ultimate chronograph, something that didn’t simply record time, but could be used as a computer.

Despite Willy’s newly founded Huit Aviation Department supplying aircraft instruments to both the civil and military aviation authorities, he refused to limit his vision to just the sky. He wanted to manufacture the ultimate chronograph, not one targeted at a single industry; as if to hammer his intentions home, he spent the 1940s making watches that had nothing to do with aviation.

These included the Premier collection, a chronograph line that was the brand’s pinnacle of quality; the Duograph, which featured a split-second complication; and the Chronomat, a patented chronograph with a turning slide-rule bezel calibrated for scientists and engineers. Chronomat: chronograph for mathematics.

Willy wasn’t shy about his handiwork, dubbing it ‘A sensational creation!’; it was, after all, effectively the smart watch of its day. With a twist of the bezel, complicated arithmetic could be made easy, from the intricate calculation of distance for an action that is both visible and audible, all the way to a more domestic reminder for long-distance phone charges.

But post-war commercial aviation boomed, and Willy was unable to ignore it any longer. Pressure from the Aircraft Owners and Pilots Association to create a specialised version of the Chronomat forced Willy to finally give in, and so he recruited mathematician Marcel Robert and the AOPA to help him convert the Chronomat’s bezel to aid navigation. His new watch, the navigation timer, or Navitimer, was announced in 1952. But how on Earth does it work?

It’s a complicated mess at first glance, but with a little bit of learning, the slide rule bezel isn’t too tricky to master. Given how unlikely it is that you’ve used a slide rule before, it’ll help to explain a few quirks of the process first. After all, in Breitling’s own 1950s instruction manual, the bezel is described as requiring ‘a little time and patience to master’. That’s not wrong.

The first quirk is fairly easy to get your head around, and that’s that the markers indicated in red are unit index markers. This means they are zeroed for different scales, serving as a reference point in the alignment of the rule.

The second quirk is a little more tricky because it involves a bit of mental juggling. The slide rule pays no heed to decimal places, and that means finding the number closest to the one you want regardless of where the decimal point sits. If you want 180 on the slide, for example, you’ll use 18, and if your answer reads 32.5, as another example, and you know it should be in the hundreds, you’ll read it as 325 instead.

So, let’s try all that in a real world example. Say you want to try your hand at some multiplication, 9 multiplied by 12, for example, here’s how you do it: align the unit index 10 on the outer scale—the reference for this calculation—to 9 on the inner. Now the scale is aligned to all multiplications of 9. All you need to do next is find 12—the number you want to multiply by 9—on the outer scale and read the answer from there: 10.8, which we know should be 108.

Dividing works too, in the reverse. For 15 divided by 12, say, we move 12 on the outer scale to 15 on the inner scale, then read the answer from the outer unit index 10: 12.5, which we know should be 1.25.

But what about the aviation stuff, the Navitimer’s party trick? Alright, how about a speed calculation: you’ve flown 104 miles in 35 minutes—so move 10.4 on the outer scale, the closest to 104 miles, to 35 on the inner scale, and you’ll get an answer at the MPH indicator at 12 o’clock of 17.8—178mph.

And, again, the formula can be rearranged, so if time is what you’re after—having travelled 486 miles at 156mph—you move 15.6 on the outer scale to MPH and read the answer at 48.6 on the outer scale: 18.7, or 187 minutes. Distance, too; 28 minutes at 148mph, 14.8 on the outer scale moves to MPH, the answer is read from 28 on the inner scale, 69 miles.

How about fuel consumption at 11.5 gallons per hour over 210 minutes? We’ve got 11.5 gallons per hour on the outer scale, we move it to MPH, we read 210 minutes on the inner scale—21 being the closest number—and we get 41 gallons. Rate of descent even, 300 feet per minute over 34 minutes: read 300 feet per minute on the outer scale as 30, move it to the inner unit index 10, and the answer from the inner scale for 34 minutes is 10.2—10,200 feet.

It’s hard to imagine flying a plane while working all this stuff out, but that’s how it was done. It was a corner of the market that Willy rather begrudgingly dominated, ironically creating the very legend of Breitling that he was trying to avoid. Still, create it he did, what is now a historic relic of a mechanical era in more ways than one. Thank goodness for computers!